Actually, there's a trick to seeing enemy Thresh souls.
When a soul is dropped, it will only appear on your screen if one of the following two statements is true for you:
- The soul was dropped an enemy unit, and you are Thresh or have a Thresh on your team, or you have personal vision of the enemy's Thresh and the unit that died
- The soul was dropped by an allied unit, and you had PERSONAL vision of both the enemy team's THRESH and the allied unit when it died, or you are Thresh and had personal vision of the unit that died.
This was explained in Thresh's Champion Spotlight.
So basically, if an allied unit dies while your personal character does not have personal vision on Thresh (remember, this excludes team vision), you will not be able to see the soul. This happens most often because Thresh is hiding in the brush while his lane buddy is getting kills, and he only comes out to collect souls.
To be able to see all of the souls that the enemy Thresh can pick up, you would need to be able to maintain PERSONAL vision on both him and dying allied/enemy units at all times. Which means warding the brush.
Again, Phreak talked about this in the Champ Spotlight... albeit very briefly.